With the extensive application of solar cell modules, photovoltaic power generation has occupied a larger and larger proportion in the new energy sources, and has developed rapidly. Among the current commercial solar cell products, crystalline silicon (single crystal and polycrystalline) solar cell has accounted the largest market share, maintaining more than 85% of the market occupancy all the time.
At present, in the solar cell production process, the textured structure of the surface of a silicon wafer can effectively reduce the surface reflectivity of solar cells, which is one of the important factors that affect the photoelectric conversion efficiency of solar cells. In order to obtain a good textured structure on the surface of the crystalline silicon solar cell to achieve a better antireflection effect, many methods have been tried, commonly including mechanical grooving method, laser etching method, reactive ion etching method (RIE), chemical etching method (i.e, wet etching method), and the like. Among them, the mechanical grooving method can obtain a lower surface reflectivity, however, this method will cause a more severe mechanical damage to the surface of the silicon wafer, and the rate of finished products thereof is relatively low, therefore it is less used in the industrial production. For the laser etching method, different grooving patterns are made with laser, and striped and inverted pyramid shaped surface have been produced, the reflectivity of which can be as low as 8.3%, but the efficiency of cells produced therefrom is low, thus it cannot be effectively used for production. RIE method can use different templates for etching, which is generally dry etching, and a so-called “black silicon” structure can be formed on the surface of the silicon wafer, the reflectivity of which can be as low as 7.9%, or even up to 4%. However, due to the expensive equipment, its production cost is high, thus its use in the industrial production is less. However, the chemical etching method has characteristics of simple process, efficient cost, high quality and good compatibility with the existing process, etc., which has become the most commonly used method in the existing industry.
At present, the textured structure of a crystalline silicon solar cell produced with the wet etching is generally in micron level. The current common practice is still to further reduce its surface reflectivity. Utility Patent Application WO2014120830(A1) discloses a method for producing a nano-textured surface of a crystalline silicon, wherein the control of the morphology of the nano-textured surface is achieved by annealing. However, this method is complicated in process and is not conducive to the needs of industrial production.
In view of the above-mentioned problems, there has emerged a metal ion etching method in the prior art, such as the Chinese patent CN101573801B, which specifically includes the steps of: (1) placing a silicon wafer in a hydrofluoric acid solution containing an oxidizing agent and a metal salt to form a porous layer structure; (2) then etching the surface with a first chemical etching solution; the first chemical etching solution is a mixed solution of hydrofluoric acid and nitric acid; (3) then impregnating the above silicon wafer in a second chemical etching solution to form a textured structure; wherein the second chemical etching solution is an alkaline solution.
However, it has been found in the practical application that the above method presents the following problems: (I) the first chemical etching solution in the step (2) of the above method has two main effects, in which one is to etch the porous silicon layer formed by metal catalysis; the other one is to clean the residual metal particles on the surface of the silicon wafer; however, as the number of the silicon wafers to be treated increases, Ag ions in the first chemical etching solution in the step (2) (i.e., a mixed solution of HF/HNO3) are increasing, so that the solution will change into a Ag ion-rich mixed solution of HF/HNO3, and the silicon wafers in this solution will occur chemical etching reaction catalyzed by metal ions again, which will affect the stability and uniformity of the textured structure, thereby influencing the electrical properties of the solar cells; (II) As the number of the silicon wafers to be treated increases, Ag ions in the first chemical etching solution in the step (2) are increasing, and the Ag ions are adhered to the silicon wafer in a reverse direction again, thus it is difficult to wash off the Ag particles attached to the silicon wafer in the first step, thereby resulting in the lifetime of the mixed solution of HF/HNO3 being very short, which further increasing the cost.
Therefore, it has a positive practical significance to develop a new method for producing a textured structure of a crystalline silicon solar cell to ensure the stability and uniformity of the textured structure, as well as the stability of the electrical properties of the solar cells, and further improve the lifetime of the mixed solution of HF/HNO3.